This invention relates to an optical measuring apparatus for measuring an absolute intensity of a light beam and, in particular, a spectrometer for measuring a spectral distribution.
A spectrometer is known as an optical measuring apparatus in which case a light beam is incident on a diffraction grating to permit it to be separated into respective light beams. Upon the rotation of the grating, the light beam components having the respective wavelengths are successively sent to photodetectors where the beam intensity of the respective wavelengths is measured in which case the photodetector is located in a specified direction to the grating. As a result, measurement is made of the spectral distribution of the light beams which has been measured by the spectrometer.
In this known spectrometer, the spectral distribution of the light beam can exactly be measured since the light beam components which have been diffracted by the diffraction grating are introduced into the photodetector. That is, a spectral element, such as a diffraction grating, has the characteristic of which the spectral efficiency differs depending upon the polarized components of the light beam. Now suppose that, for example, a light beam of a certain wavelength .lambda. having a relative output power of 10 is splitted at the diffraction grating. Even in this case, the photodetector detects a relative output power of 5 if the light beam contains more S polarization components than P polarization components and detects a relative output power of 3 if the light beam contains more P polarization components than S polarization components. This phenomenon is observed as the special characteristic of a diffraction grating shown in FIG. 1. In FIG. 1, Pa denotes a spectral characteristic for a P polarization component having a deflection plane parallel to the grooves of the diffraction grating, which Sa denotes a spectral characteristic for a P polarization component having a deflection plane perpendicular to the grooves of the diffraction grating. As evident from the characteristic graph of FIG. 1, since the conventional optical measuring apparatus is of such a type that light beam components simply diffracted at the diffraction grating are introduced directly into the photodetectors, the absolute power of the light beam cannot be precisely measured since that spectral efficiency differs depending upon the ratio between the wavelength and the deflection components of the light means.